Neutrophils, producing superoxides

Superoxide is formed (reaction 1) in the red blood cell by the auto-oxidation of hemoglobin to methemo-globin (approximately 3% of hemoglobin in human red blood cells has been calculated to auto-oxidize per day) in other tissues, it is formed by the action of enzymes such as cytochrome P450 reductase and xanthine oxidase. When stimulated by contact with bacteria, neutrophils exhibit a respiratory burst (see below) and produce superoxide in a reaction catalyzed by NADPH oxidase (reaction 2). Superoxide spontaneously dismu-tates to form H2O2 and O2 however, the rate of this same reaction is speeded up tremendously by the action of the enzyme superoxide dismutase (reaction 3). Hydrogen peroxide is subject to a number of fates. The enzyme catalase, present in many types of cells, converts... 

It is well known that neutrophils, monocytes, macrophages, and other phagocytes produce superoxide upon activation with various stimuli and therefore, are potential initiators of lipid peroxidation. In 1985, Carlin and Arfors [75,76] showed that leukocytes initiate the oxidation of unsaturated lipids. Surprisingly, the leukocyte-initiated peroxidation of linoleic acid was not inhibited by SOD and, therefore, apparently was not initiated by superoxide, while liposome peroxidation was mediated by superoxide. No convincing explanations were given. 

Systems that produce superoxide, such as human neutrophils, give a positive Ames test (34) i suggesting a connection between radical-forming activity and mutagenesis. 

Since other membranes have an integral transmembrane electron transport system, the question arises whether these electron carriers can be involved in an oxidation-reduction driven proton movement. In neutrophil as well as macrophage plasma membranes, the answer is already yes. The superoxide-producing NADPH oxidase in these membranes is associated with a channel for proton movement to accompany the electron flow when internal NADPH is oxidized by external oxygen to produce superoxide (Nanda et al., 1993). This is a relatively simple electron transport system which contains a heterodimeric cytochrome b which also binds flavin. Thus, two proteins in a transmembrane electron transport system can transfer protons across the membrane. 

Activation of PKC is supposed to be a necessary prerequisite for neutrophil respiratory burst, because it mediates the phosphorylation of the proteins involved in the assembly of NADPH oxidase, which at last produces superoxide anion (02 ) from external oxygen. The isoprenylflavone cycloheterophyllin (12) isolated from Artocarpus heterophyllus (Moraceae) was known to inhibit that respiratory burst, and its interaction with PKC was therefore studied. 

Polymorphonuclear leukocytes (PMN or neutrophils) participate in the systemic immune system. Neutrophils are an integral part of the body s defense mechanism against pathogens and, upon activation, are recmited to the site of injury, where they produce superoxide anions Via the NADPH oxidase pathway. This enzyme is a multiprotein complex that is assembled upon PMN activation, and it reduces molecular oxygen (O2) to the superoxide anion Under inflammatory conditions, neutrophils also generate nitric oxide by both constitutive (nNOS) and inducible (iNOS) nitric oxide... 

Historically, leukocyte NADPH oxidase was the first discovered enzyme of this type the existence of this enzyme in neutrophils was reported in 1962-1966 [59-61]. (As the evidence of superoxide production in biological systems has been obtained several years later, at that time the nature of oxygen species produced by leukocytes was of course unknown.) And only about 10 years later, Babior et al. [62] have shown that the activation of human neutrophils resulted in the production of superoxide. The structure of leukocyte NADPH has been widely discussed and well-established [57]. Superoxide production catalyzed by NADPH oxidase is described by Reaction (5) ... 

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